Method of making ceramic multilayer circuit boards mounted in a patterned metal support substrate

ABSTRACT

A package for an electronic component includes a metal support substrate having a pattern of openings therethrough and a body of an insulating material, such as glass or ceramic, on and bonded to the surface of the support substrate. The body is formed from a plurality of layers of an insulating material, and conductive vias extending through the plurality of layers to the support substrate; said insulating body having an opening therein, an electronic component directly mounted in said opening to the patterned base plate. The base plate can be cut into one or more modules and directly soldered to a motherboard having additional devices mounted thereon.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 09/852,901, filed on May 10, 2001 (now U.S. Pat. No. 6518,502,issued on Feb. 11, 2003).

This invention relates to multilayer ceramic circuit boards mounted on apatterned conductive substrate support and method of making. Moreparticularly, this invention relates to mounting modules of high poweramplifiers and oscillators onto the patterned substrate support toremove excess thermal energy from these devices.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,847,935 to Thaler et al describes an electronic circuitchip package which includes a base plate of an electrically and/orthermally conductive material, such as a metal plate, having a body ofan insulator adhered to the base plate. This insulator body may comprisea plurality of glass-ceramic layers having printed circuitry thereon.Conductive vias through the ceramic layers connect the circuitselectrically to each other. At least one opening is formed in theinsulator body through to the support substrate, the opening having asize so as to accommodate at least one electronic device or module. Theelectronic device is mounted directly onto the conductive base plate.Wires are used to connect terminals in the electronic device to terminalstrips in the circuitry.

This technology is known as low temperature co-fired ceramic circuitboards with metal support substrates, or LTCC-M technology. Thistechnology permits the inclusion of active and passive devices betweenthe glass-ceramic layers, as well as the inclusion of transmissionlines, capacitors, resistors and the like.

When mounting high power amplifier and oscillator electronic devices toa substrate, a common problem is that of removing excess thermal energyfrom the devices, so that the active devices and circuit components aremaintained at operating temperatures consistent with reliableperformance.

The metal substrate support of the above-described LTCC-M technology canserve also as a heat spreader, or heat sink, which is advantageous inthat modules can be directly mounted to the metal substrate such thatthey have an improved heat energy path. Thus by mounting high powercomponents, such as amplifiers and oscillators, in an opening throughthe glass-ceramic layers directly onto a metal substrate support,minimal heat resistance to the heat sink can be achieved. The failurerate (MTTF) of power amplifiers and oscillators, as well as other highpower devices, can thus be reduced markedly.

However, it would be desirable to be able to surface mount the aboveLTCC-M modules to a motherboard for increased integration. In order todo that, the signal-power leads can be desirably soldered directly toappropriate contacts on the motherboard. Such a configuration requiresthat the power and signal leads of the module are brought through theheat sink. Such a configuration would improve reliability and reducemanufacturing costs. However, forming openings through the metal supportsubstrate in a completed LTCC-M device is expensive and presents variousproblems of manufacturability. Thus heat sinked modules for high powerdevices that can be readily made at low cost would be highly desirable.

SUMMARY OF THE INVENTION

We have found that by patterning the metal support in an LTCC-M moduleso as to be able to provide openings through the metal support prior tomounting one or more high power modules thereon, and applying solderpads to both sides of the support, the power and the signal leads ofdevices thereon, and heat sinking of the module, can all be achieved.Through vias in the ceramic layers of the LTCC-M module are directlyconnected to the top side of the metal support substrate, which alsoacts as a heat sink for a high power module.

Another aspect of the invention includes a dielectric paste that isscreen printed onto the bottom side of the LTCC-M module so that thedielectric paste is aligned with the openings in the metal substratesupport. This dielectric paste prevents shrinkage in the x and ydimensions of the green tapes mounted in those areas that overlie theopenings in the metal substrate. After laminating the prepared module tothe prepared substrate, and firing, the dielectric, now a powder, isreadily removed.

This design further permits high integration because the above heat sinkand module and other circuitry can be directly soldered from the bottomside of the substrate support to solder pads on a motherboard, which caninclude additional devices.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a bottom view of an example of a high power module of theinvention.

FIG. 2 is a schematic layout of a plurality of high power modules of theinvention that can be cut into single modules.

FIG. 3 is a cross sectional view of a multilayer ceramic tape with apatterned metal support substrate of the invention.

FIG. 4 illustrates a cross section of a power module of the inventionsoldered to a motherboard.

DETAILED DESCRIPTION OF THE INVENTION

The metal support substrate is prepared by first forming openingsthrough the metal support, as by cutting, laser cutting, etching,electro discharge machining or punching openings to permit electricalconnection of devices mounted on one side (the top side) of the metalsubstrate to the bottom side. These openings provide through connectionof all of the devices built onto the top side of the metal support tothe bottom side of the metal support.

After patterning the support substrate, it is nickel plated. A solderpaste is then screen printed to both sides of the metal support, using asuitable screen printable conductive solder ink. A suitablesilver-palladium screen printable solder ink can be made by mixing 20parts by weight of silver or silver-palladium powders, such as oneavailable from Degussa Corp. as K!P 3030; 0.6737 part by weight of aglass powder having the following composition: 103.72 parts by weight ofzinc oxide powder; 103.0 parts by weight of magnesium oxide powder; 78.2parts by weight of boric anhydride, all available from J. T. Baker;115.08 parts by weight of silicon oxide powder, 20.0 parts by weight ofaluminum oxide, and 01036 part by weight of boron powder having aparticle size of 0.8 micron. This glass composition is mixed with anorganic vehicle made by mixing 0.33 part by weight of Hypermer PS 2dispersant from ICI Americas, Inc; and 3.56 parts by weight of an ethylcellulose-solvent mixture comprising 19.4 parts by weight of ethylcellulose having a molecular weight of 300; 18.8 parts by weight of anethyl cellulose having a molecular weight of 14; 133 parts by weight ofbutyl carbitol; 88.8 parts by weight of dodecanol; and 0.2 part byweight of lecithin to form a screen printable ink. The same organicvehicle can also be used for the dielectric paste.

The support substrate is then fired in nitrogen at temperatures of about750-800° C. to densify the solder ink.

The support substrate is then oxidized by heating in air at atemperature of about 800-850° C. A glaze is screen printed on one sideof the patterned substrate, in the same pattern as the substrate. Theglazed substrate is fired again in air at a temperature of about 750° C.to densify the glaze. The glaze ink acts to adhere components to the topside of the substrate support. An organic binder and solvent adhesionpromoter can be applied to the top side of the substrate support foradditional adhesion of mounted components.

After preparation of the support substrate as above, a multilayer greentape stack, including circuitry and embedded devices as desired, andmade of a glass composition that is thermally matched to the supportsubstrate, is screen printed on the side to be adhered to the supportsubstrate with a high temperature dielectric paste, in a pattern that isthe mirror image of the pattern of openings in the support substrate.Thus, when the green tape stack is aligned with the support substrate,the dielectric paste covers the openings in the substrate. Thisdielectric paste prevents x and y shrinkage in the overlying green tapesduring firing in areas that are exposed, i.e., areas over the openingsin the substrate support, that would be otherwise subject to suchshrinkage.

Suitable inert and stabilizing materials for the dielectric pastecomprise a high sintering temperature oxide of a metal, such as aluminaor zirconia, having a higher firing temperature than the green tapes.These dielectrics are mixed with organic materials, such as organicresins and solvents, to form a screen printable paste.

A suitable high temperature dielectric paste comprises 20.0 parts byweight of alumina powder having a median particle size of about 40microns, 16 parts by weight of an organic vehicle and 0.2 parts byweight of lecithin.

The prepared metal support and the laminated green tape stack are thenco-laminated under heat and pressure by applying a polyethyleneterephthalate tape over the green tapes. The green tapes are mounted andaligned so that the pattern in the green tapes corresponds to theopenings in the metal support, now filled with dielectric or stabilizingpaste., and placed between two copper plates. The resulting sandwich isheated to about 200° F. and pressed in a uniaxial press using 750 poundsof force for about two minutes.

The co-laminate is then fired in a belt furnace at a temperature of fromabout 850-900° C. Upon firing, the organic materials are vaporized, theglasses are densified, and the dielectric paste is now a powderedmaterial that can be readily removed from the openings in the metalsupport substrate by brushing, or by immersing in an ultrasonic bath, asof isopropyl alcohol.

Alternatively, the dielectric or stabilizing paste can be applied to theceramic after the co-lamination step by squeezing or doctor blading thepaste into the openings in the metal support from the backside of theco-laminate, or by dispensing the paste into the openings with asyringe.

One of more openings can be made in the ceramic layer for insertion of apower module directly mounted to the metal support substrate. Thismodule can be soldered onto a motherboard which may include otherdevices.

FIG. 1 is a bottom view of a high power module of the invention. Thedark areas 10 illustrate the patterned metal substrate, and the lightareas 12 illustrate edges of the ceramic module.

FIG. 2 is a schematic layout of a patterned metal support substrate. Thelight areas 20 are those in which metal will be removed, as by punching.The darker areas 22 show the remaining metal support substrates. Aftercompletion of the modules, they can be separated by sawing along thedashed lines 24.

FIG. 3 is a cross sectional view of a multilayer ceramic tape 30 on apatterned metal support substrate 32. The metal support 32 has anopening 34 punched through it, which is at least partially filled with ahigh melt temperature inert dielectric or stabilization layer 36. Thestabilization layer 36 prevents shrinkage of the green tape layerssituated over the opening 34 in the x and y directions during firing.

The metal substrates are then cut into individual modules, as shown inFIG. 2. These modules have isolated pads along their periphery and acentral ground plane which can be directly soldered to a motherboardhaving suitable bond pads thereon. This method forms low inductance-highspeed RF connections to the motherboard. The metal support substrateprovides good heat dissipation for the modules.

FIG. 4 illustrates a patterned metal support substrate 40 havingopenings 42, 44 (pad openings) therein from which the high melttemperature dielectric 36 has been removed. Multilayer ceramic circuits46 are mounted on the patterned metal support substrate 40. Conductivevias 48 through the ceramic layers connect the circuitry 46 to thepatterned metal support 40. An opening 50 formed in the ceramic layers46 provides for mounting a power amplifier die 52 directly onto thepatterned metal support 40. This die 52 can be connected to themultilayer ceramic circuitry 46 by means of wire bonds 54.

The resultant module 56 is mounted on a motherboard 58 so that solderconnections on the bottom side of the metal support are aligned withsolder connections 60 made on the motherboard 58. The module 56 is thuselectrically connected to the motherboard 58.

The invention will be further described in the following examples, butthe invention is not meant to be limited by the details describedtherein. Other glasses, resins, dielectric powders, metal supportsubstrates and the like can be substituted, as will be well known tothose skilled in the art.

EXAMPLE 1 Preparing a Patterned Metal Substrate

A copper clad molybdenum sheet about 10 mils in thickness was cut intosupport substrates 2″ by 2″. Each substrate was electro-dischargemachined with a hole pattern, such as that shown in FIG. 2. Thesubstrates were then electroplated with nickel to a coating thickness ofabout 1 mil. A solder paste was screen printed onto both sides of thesupport substrate and fired in nitrogen at 750-800° C. and oxidized bypassing through an air fired furnace having a peak temperature of about800-850° C. One side was glazed in a pattern the same as the metalpattern. The glazed substrate were then heated in an air fired beltfurnace to a maximum temperature of 750° C. to densify the glass. A gluecomposition comprising organic binders and solvent was screen printedover the fired glaze layer.

A two layer ceramic tape comprising a glass thermally matched to themetal substrate support was screen printed on one side with an inert orstabilizing dielectric paste, such as that described above, in a patternmatching the openings in the metal substrate.

A co-laminate was made by placing the pre-cut two layer ceramic tapeover the substrate, adding a Mylar film on top, sandwiching the tape andmetal support between two copper plates and co-laminating by applyingpressure in a uniaxial press at 200° F. using 750 lbs of force for twominutes. A dielectric tape was co-laminated to the bottom of the metalsupport in the same or separate co-lamination step.

The dielectric pattern on the green tape was aligned with the openingsin the metal support substrate. The stabilizing dielectric paste presentin the openings of the metal core constrains shrinkage during firing ofthose portions of the green tape which are not in direct contact withthe metal core, and thus which are otherwise free to shrink in all x, yand z directions, causing tearing of the green tapes adjacent to theopenings in the substrate.

EXAMPLE 2 Assembling a Module of Example 1 to a Motherboard

The fired ceramic layers adhered to the patterned metal supportsubstrate have one or more openings of a size for directly mounting oneor more power modules, such as a power amplifier or oscillator, to themetal support substrate. These power modules can be connected to thecircuitry in the ceramic layers by means of wire bonds. The metalsupport substrate acts as a heat sink.

The powder remaining in the openings in the metal support substrate isreadily removed by brushing or by immersing in an ultrasonic bath of,for example, isopropyl alcohol.

After cutting the substrates to form individual modules having isolatedpads about their periphery and a central ground plane, the power modulescan be soldered to a motherboard directly by solder connecting tosuitable pads on the motherboard. Other device die also can be directlymounted to the metal support substrate through suitable openings in theceramic layers in like manner. This eliminates the need for lead framesand pins to make connections between devices.

Suitably vias through the multilayer ceramic layers are made of silverto electrically connect the top surface of the ceramic circuits and/orconductors to the bottom side signal and/or ground pads formed in themetal support substrate by the patterning and cutting operations.

Although the invention has been described in terms of specificembodiments, the invention is not meant to be limited to the detailsdescribed herein, but only by the scope of the appended claims.

We claim:
 1. A method of forming a high power device module comprising a) forming a pattern of pad openings completely through the metal support substrate; b) electroplating said support substrate with nickel; c) screen printing a patterned conductive solder ink onto both sides of said support substrate; d) firing said support substrate in nitrogen; e) oxidizing the nickel layer by heating in air; f) applying a glazing layer onto the top of said support substrate; g) mounting a multilayer green tape stack having circuitry on each green tape onto the glazed surface of the support substrate; h) laminating the green tape stack to the patterned support substrate; i) filling the openings in the metal support substrate with a high melt temperature dielectric stabilization paste; and j) firing said module.
 2. A method according to claim 1 wherein the dielectric stabilization paste comprises a metal oxide selected from the group consisting of aluminum oxide, zirconium oxide and a mixture of aluminum oxide and zirconium oxide, together with an organic vehicle.
 3. A method according to claim 1 wherein an opening is made in the multilayer ceramic stack through to the support substrate.
 4. A method according to claim 3 wherein a high power electronic device is mounted in said opening and connected to the multilayer stack circuitry.
 5. A method according to claim 4 wherein the circuits on the multilayer stack are connected through to the support substrate with silver.
 6. A method according to claim 5 wherein the module is soldered to a motherboard. 